1. Field of the Invention
This invention relates generally to interface circuits. More particularly, it relates to an optical interface which has improved transmitter and receiver circuitry that provides improved performance, reduces transmission errors due to timing jitter, that provides improved noise immunity and is suitable for use with low level signals, and that is particularly well suited for use in multi-tier local area networks.
2. Description of the Related Art
At present, two primary technologies are employed in applications requiring the local transmission of data: conventional hardwire technology and relatively new fiber optic technology. With hardwire technology, data is normally transmitted in the form of RF modulated data signals. The primary advantage of hardwire technology is that it can be used over relatively long distances with acceptable signal loss. Improved fiber optic cables are also providing increased distance and reduced signal loss in the fiber optic technology.
In addition, fiber optic technology has certain advantages over hardwire technology that make it desirable in many applications. For example, the fiber optic technology is not affected by certain types of electrical and magnetic interference that affect hardwire communications. Also, fiber optic technology provides increased security since, unlike the signal on a hardwire cable, the signal on a fiber optic cable cannot be tapped off or diverted between the data transmitting source and the receiver without the interception being recognized at the receiver.
One problem that has affected both hardwire and fiber optic technology in some applications, for example multi-tier local area networks, is susceptibility to transmission errors due to timing jitter caused by the repeated reception and re-transmission of data signals. In large multi-tier networks with many nodes, such timing jitter results in inordinate numbers of transmission errors which may increase the average transmission time and hence adversely affect the efficiency and throughput of the network. In severe cases, the loss of efficiency may be so great as to perhaps even render the desired application impracticable.
In the past, it has been suggested to buffer the received data or to employ phase locked loop receivers in order to overcome the effects of timing jitter. However, these approaches have not proved entirely satisfactory. The buffer approach limits the capacity and speed of transmissions because the buffers obviously have finite length and because additional time is required to read out the received data. The phase locked loop approach also limits the efficiency of the network, and particularly high speed networks, because it requires a relatively long time to lock on to the input data signal. Accordingly, an undesirably long preamble or locking signal is typically required to precede the actual data signals.
In applications where signals are transmitted at relatively low levels, noise interference has also been a problem. Typical receiver circuits have employed pulse detectors that detect predetermined signal threshold or zero crossings. Such receivers may not provide adequate noise immunity where low level signals are to be transmitted in a noisy environment.
In view of the foregoing, it is an object of the present invention to provide an improved optical interface that provides improved performance and noise immunity, that decreases the occurrence of transmission errors due to timing jitter, and that exhibits none of the aforementioned drawbacks of the prior art approaches.
More particularly, it is an object of the invention to provide such an interface that is particularly suited for use in applications involving the local transmission of data, such as local area networking.